By way of brief background, Metal Inert Gas (MIG) welding employs a continuously fed wire electrode (consumable) and a shielding gas. Generally, the wire consumable is fed through a torch to the workpiece. The latter is connected by an earth lead which completes the circuit. The wire electrode is held at a potential above ground using a power source capable of supplying several hundred amps of current to produce an arc. When the wire consumable touches the workpiece, an arc is formed which melts localized metal of the workpiece and the wire, forming a molten pool which forms the weld. The shielding gas, usually an argon/CO.sub.2 mixture, is also supplied through the torch and protects the weld from oxidation and provides the desired arc characteristics.
Up until 1989, MIG welding in steam turbine diaphragms required 22.degree. weld prep angles in order for the traditional weld cone to access the bottom of the weld and to provide adequate gas shielding. In other words, the access groove required a 22.degree. angle to enable the welding torch to reach the weld area. Although a good quality weld resulted, this led to excessive weld material being deposited into the diaphragm, increasing the product's distortion, cycle and cost.
In 1989, an adjustable copper sheath was developed which extended from 1" to 8" beyond the original end of the cone. This configuration was developed in order to reduce the weld prep angle from 22.degree. to 11.degree.. This significantly reduced the weld volume and direct costs for each diaphragm, but still left substantial room for improvement.
A concept of narrow prep welding known as "Fine Line Welding" was recently introduced in connection with Tungsten Inert Gas (TIG) welding. Although this technology would certainly reduce the distortion found in standard diaphragms, it has two qualities which make it inappropriate for diaphragm production use, namely, a) the machinery currently being used for this application is very delicate, still in the development stage and not rugged enough for the manufacturing environment and b) the material deposition rate is too low to be satisfactory for diaphragm structural welding. The development of a weld process that permitted even smaller weld prep angles consequently focused on modification of existing MIG welding techniques.
Weld distortion associated with traditional MIG welding apparatus has typically resulted in unfavorable:
1) Steam path dimensional deviations between before and after welding conditions; PA1 2) Circumferential shrinkage of the diaphragm halves leading to poor joint blade matching in subsequent finishing operations; PA1 3) Excessive manual deburring of steam path configuration which had been distorted due to welding; PA1 4) Weld splatter as a result of excess weld and weld buildups required for access to traditional sized preps; and PA1 5) Distortion of radial location of steam path sidewalls and setbacks to the trailing edges of the blades. PA1 1) Welding and cleaning of the structural and cover welds of the diaphragm, PA1 2) Welding of diaphragm appendage rings and outer ring buildups required to allow access to the traditional sized weld preps, PA1 3) Use of excess shielding gas and filler material in welding larger weld preps, PA1 4) Machining time for weld preps and appendage rings, PA1 5) Material costs for appendage rings.
Labor costs associated with MIG welding in the turbine environment are also high. Examples of operations which drive these costs in a steam turbine diaphragm welding application are,